Network handover method, device, and system

ABSTRACT

In a network handover method, after UE located on an LTE network initiates a request message that requests CSFB, an MME instructs an enodeB to move the UE from the LTE network to a 2G or 3G network, and requests an MSC to hand over the UE from the LTE network to a CS domain of the 2G or 3G network for the CSFB, so that the 2G or 3G network allocates a CS domain resource to the UE. Further, by determining that an IMS voice session does not exist, it is identified that the handover of the UE from the LTE network to the CS domain of the 2G or 3G network is triggered by the CSFB, so as to ensure that handover from the LTE network is properly performed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/142,455, filed on Apr. 29, 2016, now U.S. Pat. No. 9,826,440 which isa continuation of International Application No. PCT/CN2013/086452, filedon Nov. 1, 2013. All of the aforementioned applications are herebyincorporated by reference in their entireties.

TECHNICAL FIELD

The present invention relates to the field of communicationstechnologies, and specifically, to a network handover method, device,and system.

BACKGROUND

Full coverage of a second generation (2G) or third generation (3G)network, such as Global System for Mobile Communications (GSM) orWideband Code Division Multiple Access (WCDMA) has been basicallyimplemented.

With development of Long Term Evolution (LTE) network technologies, LTEnetworks have covered some urban areas and traffic hot spot areas. Inthis way, on current communications networks, the LTE networks coexistwith the 2G or 3G network.

During a call, user equipment (UE) may enter a 2G or 3G network from anLTE network, and handover from a packet switched (PS) domain to acircuit switched (CS) domain is required due to different bearermechanisms of the LTE network and the 2G or 3G network, whereinterruption of a voice call may occur, and consequently, continuity ofa voice call of a user is affected.

Circuit switched fallback (CSFB) is that when UE covered by LTE networkprocesses a voice service, the UE first falls back to a network that hasa CS domain, and processes the voice service on the network that has theCS domain, thereby fulfilling an objective of reusing an existing CSdomain device to provide a traditional voice service for a user on theLTE network. In the prior art, the UE needs to be handed over from theLTE network to a PS domain of the 2G or 3G network, or to be redirectedfrom the LTE network to the 2G or 3G network, and then initiates, on the2G or 3G network, a connection to the CS domain. In the prior art, eventhough the UE and the network can support handover from the LTE networkto the CS domain of the 2G or 3G network, the UE needs to be moved tothe PS domain of the 2G or 3G network so as to initiate a voice call,which causes relatively long duration of an entire voice call and pooruser experience.

SUMMARY

To resolve the foregoing problem in the prior art, embodiments of thepresent invention provide a network handover method, and in a process ofimplementing CSFB, handover of UE from an LTE network to a CS domain ofa 2G or 3G network is triggered, thereby reducing an access delay ofaccessing the CS domain of the 2G or 3G network by the UE, and alsoimproving user experience. In addition, by determining that an IMS voicesession does not exist, it is identified that the handover of the UEfrom the LTE network to the CS domain of the 2G or 3G network istriggered by the CSFB, so as to ensure that an enodeB can still triggerthe handover from the LTE network to the CS domain of the 2G or 3Gnetwork for the CSFB according to a handover capability of the LTE in acase in which there is no voice bearer whose QCI=1. In addition, it isensured that an MME/MSC can properly perform the handover from the LTEnetwork to the CS domain of the 2G or 3G network in a case in whichthere is no IMS voice session. The embodiments of the present inventionfurther provide a corresponding device and system.

A first aspect of the present invention provides a network handovermethod, including:

receiving, by a mobility management entity MME, a request message sentby user equipment UE located on a Long Term Evolution LTE network, wherethe request message is used to request circuit switched fallback CSFB;

sending, by the MME, a notification message to an evolved NodeB eNodeBaccording to the request message, where the notification message is usedto instruct the eNodeB to move the UE from the LTE network to a circuitswitched CS domain of a second generation 2G or third generation 3Gnetwork for the CSFB:

receiving, by the MME, a handover requirement message sent by theeNodeB;

determining, by the MME, that an IMS voice session of the UE does notexist and determining not to delete a bearer corresponding to the IMSvoice session; and

sending, by the MME, a first handover request message to a mobileswitching center MSC, where the first handover request message is usedto request the MSC to hand over the UE from the LTE network to the CSdomain of the 2G or 3G network for the CSFB, so that the 2G or 3Gnetwork allocates a CS domain resource to the UE, and the CS domainresource is used to hand over the UE from the LTE network to the CSdomain of the 2G or 3G network.

With reference to the first aspect of the present invention, a firstimplementation manner of the first aspect of the present inventionincludes:

the notification message is further used to indicate that the LTEnetwork, the 2G or 3G network, and the UE support the handover of the UEfrom the LTE network to the CS domain of the 2G or 3G network for theCSFB; or

the handover requirement message is further used to indicate that theLTE supports the handover from the LTE network to the CS domain of the2G or 3G network; or

the first handover request message is further used to indicate that theUE supports the handover from the LTE network to the CS domain of the 2Gor 3G network.

With reference to the first implementation manner of the first aspect ofthe present invention, a second implementation manner of the firstaspect of the present invention includes: the notification messageincludes indication information, where the indication information isused to indicate that the LTE network, the 2G or 3G network, and the UEsupport the handover of the UE from the LTE network to the CS domain ofthe 2G or 3G network for the CSFB.

With reference to the second implementation manner of the first aspectof the present invention, a third implementation manner of the firstaspect of the present invention includes: the indication information isused to indicate that the LTE network, the 2G or 3G network, and the UEsupport single radio voice call continuity SRVCC handover.

With reference to the first aspect of the present invention, the firstimplementation manner of the first aspect of the present invention, thesecond implementation manner of the first aspect of the presentinvention, or the third implementation manner of the first aspect of thepresent invention, a fourth implementation manner of the first aspect ofthe present invention includes:

determining, by the MME, that the LTE network and the 2G or 3G networksupport the handover of the UE from the LTE network to the CS domain ofthe 2G or 3G network for the CSFB and the UE supports the handover fromthe LTE network to the CS domain of the 2G or 3G network.

With reference to the first aspect of the present invention, the firstimplementation manner of the first aspect of the present invention, thesecond implementation manner of the first aspect of the presentinvention, the third implementation manner of the first aspect of thepresent invention, or the fourth implementation manner of the firstaspect of the present invention, a fifth implementation manner of thefirst aspect of the present invention includes:

receiving, by the MME, a first handover response message that is sent bythe MSC to respond to the first handover request message, where thefirst handover response message includes information about a CS domainradio resource allocated by a base station on the 2G or 3G network tothe UE; and

sending, by the MME, a handover requirement response message to theeNodeB, where the handover requirement response message includes theinformation about the CS domain radio resource, and the informationabout the CS domain radio resource is used to hand over the UE from theLTE network to the CS domain of the 2G or 3G network for the CSFB.

With reference to the first aspect of the present invention, the firstimplementation manner of the first aspect of the present invention, thesecond implementation manner of the first aspect of the presentinvention, the third implementation manner of the first aspect of thepresent invention, the fourth implementation manner of the first aspectof the present invention, or the fifth implementation manner of thefirst aspect of the present invention, a sixth implementation manner ofthe first aspect of the present invention includes:

the handover requirement message is a message for requiring handoverfrom the LTE network to a packet switched PS domain of the 2G or 3Gnetwork, or a message for requiring the handover from the LTE network tothe CS domain of the 2G or 3G network.

With reference to the first aspect of the present invention, the firstimplementation manner of the first aspect of the present invention, thesecond implementation maimer of the first aspect of the presentinvention, the third implementation maimer of the first aspect of thepresent invention, the fourth implementation maimer of the first aspectof the present invention, or the fifth implementation maimer of thefirst aspect of the present invention, a seventh implementation maimerof the first aspect of the present invention includes:

the handover requirement message is a message for requiring handoverfrom the LTE network to a PS domain and the CS domain of the 2G or 3Gnetwork.

With reference to the seventh implementation manner of the first aspectof the present invention, an eighth implementation maimer of the firstaspect of the present invention includes:

sending, by the MME, a second handover request message to a servinggeneral packet radio service support node SGSN, where the secondhandover request message is used to request the SGSN to hand over the UEfrom the LTE network to the PS domain of the 2G or 3G network for theCSFB.

With reference to the first aspect of the present invention, the firstimplementation manner of the first aspect of the present invention, thesecond implementation manner of the first aspect of the presentinvention, the third implementation manner of the first aspect of thepresent invention, the fourth implementation manner of the first aspectof the present invention, the fifth implementation manner of the firstaspect of the present invention, the sixth implementation manner of thefirst aspect of the present invention, the seventh implementation mannerof the first aspect of the present invention, or the eighthimplementation manner of the first aspect of the present invention, aninth implementation manner of the first aspect of the present inventionincludes:

the bearer corresponding to the IMS voice session is a bearer whose QoSclass identifier equals to 1.

A second aspect of the present invention provides a network handovermethod, including:

receiving, by an evolved NodeB eNodeB located on a Long Term EvolutionLTE network, a notification message sent by a mobility management entityMME, where the notification message is used to instruct the eNodeB tomove user equipment from the LTE network to a circuit switched CS domainof a 2G or 3G network for circuit switched fallback CSFB:

determining, according to a capability of the UE, that the UE supportsthe handover from the LTE network to the CS domain of the 2G or 3Gnetwork;

determining, by the eNodeB, that an IMS voice session of the UE does notexist, and sending, by the eNodeB, a handover requirement message to theMME;

receiving, by the eNodeB, a handover requirement response message sentby the MME, where the handover requirement response message includesinformation about a CS domain radio resource, and the information aboutthe CS domain radio resource is used to hand over the UE from the LTEnetwork to the CS domain of the 2G or 3G network; and

sending, by the eNodeB, a handover command to the UE, where the handovercommand is used to instruct that the UE be handed over from the LTEnetwork to the CS domain of the 2G or 3G network, and the handovercommand includes the information about the CS domain radio resource.

With reference to the second aspect of the present invention, a firstimplementation manner of the second aspect of the present inventionincludes:

the notification message is further used to indicate that the LTEnetwork, the 2G or 3G network, and the UE support the handover of the UEfrom the LTE network to the CS domain of the 2G or 3G network for theCSFB; or

the handover command is used to instinct that the UE be handed over fromthe LTE network to the circuit switched CS domain of the 2G or 3Gnetwork for the CSFB.

With reference to the second aspect of the present invention or thefirst implementation manner of the second aspect of the presentinvention, a second implementation manner of the second aspect of thepresent invention includes:

the determining, by the eNodeB, that an IMS voice session of the UE doesnot exist includes:

determining, by the eNodeB by determining that a bearer whose QoS classidentifier equals to 1 does not exist in the UE, that the IMS voicesession of the UE does not exist.

With reference to the second aspect of the present invention, the firstimplementation manner of the second aspect of the present invention, orthe second implementation manner of the second aspect of the presentinvention, a third implementation manner of the second aspect of thepresent invention includes:

the handover requirement message includes first indication information,where the first indication information is used to request the MME tohand over the UE from the LTE network to the CS domain of the 2G or 3Gnetwork for the CSFB: or

the first indication information is used to request the MME to hand overthe UE from the LTE network to the CS domain of the 2G or 3G network forthe CSFB and is used to indicate that the UE supports the handover fromthe LTE network to the CS domain of the 2G or 3G network; or

the handover requirement message includes a source-to-target transparentcontainer, where the source-to-target transparent container includessecond indication information, and the second indication information isused to request a base station on the 2G or 3G network to hand over theUE from the LTE network to the CS domain of the 2G or 3G network for theCSFB.

With reference to the third implementation manner of the second aspectof the present invention, a fourth implementation manner of the secondaspect of the present invention includes:

the second indication information is further used to indicate that theUE supports the handover from the LTE network to the CS domain of the 2Gor 3G network.

With reference to the second aspect of the present invention, the firstimplementation manner of the second aspect of the present invention, thesecond implementation manner of the second aspect of the presentinvention, the third implementation manner of the second aspect of thepresent invention, or the fourth implementation manner of the secondaspect of the present invention, a fifth implementation manner of thesecond aspect of the present invention includes:

the handover requirement message is a message for requiring handoverfrom the LTE network to a packet switched PS domain of the 2G or 3Gnetwork, or a message for requiring the handover from the LTE network tothe CS domain of the 2G or 3G network.

With reference to the second aspect of the present invention, the firstimplementation manner of the second aspect of the present invention, thesecond implementation manner of the second aspect of the presentinvention, the third implementation manner of the second aspect of thepresent invention, or the fourth implementation manner of the secondaspect of the present invention, a sixth implementation manner of thesecond aspect of the present invention includes:

the handover requirement message is a message for requiring handoverfrom the LTE network to a PS domain and the CS domain of the 2G or 3Gnetwork.

With reference to the sixth implementation manner of the second aspectof the present invention, a seventh implementation manner of the secondaspect of the present invention includes:

determining, by the eNodeB, that the UE supports handover from the LTEnetwork to the PS domain of the 2G or 3G network, where

the handover command is further used to instruct that the UE be handedover from the LTE network to the PS domain of the 2G or 3G network, thehandover command includes information about a PS domain resource of the2G or 3G network, and the information about the PS domain resource isused to hand over the UE from the LTE network to the PS domain of the 2Gor 3G network.

A third aspect of the present invention provides a network handovermethod, including:

receiving, by a mobile switching center MSC, a first handover requestmessage sent by a mobility management entity MME, where the firsthandover request message is used to request the MSC to hand over userequipment UE from a Long Term Evolution LTE network to a circuitswitched CS domain of a second generation 2G or third generation 3Gnetwork for circuit switched fallback CSFB;

determining, by the MSC according to the first handover request message,not to initiate a session transfer process to an IP multimediasubsystem;

sending, by the MSC, a third handover request message to a base stationon the 2G or 3G network, where the third handover request message isused to request the base station to allocate a CS domain resource to theUE for the handover to the CS domain of the 2G or 3G network;

receiving, by the MSC, a third handover request response message sent bythe base station, where the third handover request response messageincludes information about a CS domain radio resource allocated by thebase station to the UE for the handover to the CS domain of the 2G or 3Gnetwork;

sending, by the MSC, a first handover response message to the MME, wherethe first handover response message includes the information about theCS domain radio resource; and

receiving, by the MSC, a handover complete message sent by the UE, wherethe handover complete message indicates that the UE has alreadycompleted the handover from the LTE network to the CS domain of the 2Gor 3G network.

With reference to the third aspect of the present invention, a firstimplementation manner of the third aspect of the present inventionincludes: the first handover request message is further used to indicatethat the UE supports the handover from the LTE network to the CS domainof the 2G or 3G network.

With reference to the third aspect of the present invention or the firstimplementation manner of the third aspect of the present invention, asecond implementation manner of the third aspect of the presentinvention includes: receiving, by the MSC, a first CS call connectionsetup request message sent by the UE, where the first CS call connectionsetup request message is used to request the MSC to generate a firstsession.

With reference to the third aspect of the present invention or the firstimplementation manner of the third aspect of the present invention, athird implementation manner of the third aspect of the present inventionincludes:

sending, by the MSC, a second CS call connection setup request messageto the UE, where the second CS call connection setup request message isused to request the UE to generate a second session.

With reference to the second implementation manner of the third aspectof the present invention, a fourth implementation manner of the thirdaspect of the present invention includes:

sending, by the MSC after receiving the first CS call connection setuprequest message, a session release message to the UE, where the sessionrelease message is used to request the UE to release a third sessiongenerated by the UE un a process of the handover from the LTE network tothe CS domain of the 2G or 3G network.

With reference to the third implementation manner of the third aspect ofthe present invention, a fifth implementation manner of the third aspectof the present invention includes:

sending, by the MSC after receiving a call acknowledgement message thatresponds to the second CS call connection setup request message, asession release message to the UE, where the session release message isused to request the UE to release a third session generated by the UE una process of the handover from the LTE network to the CS domain of the2G or 3G network.

A fourth aspect of the present invention provides a network handovermethod, including:

sending, by user equipment UE located on a Long Term Evolution LTEnetwork, a request message to a mobility management entity MME, wherethe request message is used to request circuit switched fallback CSFB:

receiving, by the UE, a handover command sent by an evolved NodeBeNodeB, where the handover command is used to instruct that the UE behanded over from the LTE network to a circuit switched CS domain of a 2Gor 3G network for the CSFB, and the handover command includesinformation about a CS domain radio resource allocated by the 2G or 3Gnetwork to the UE for the handover from the LTE network to the CS domainof the 2G or 3G network:

handing over the UE from the LTE network to the CS domain of the 2G or3G network according to the handover command: and

generating, by the UE, a session after the UE is handed over from theLTE network to the CS domain of the 2G or 3G network.

With reference to the fourth aspect of the present invention, a firstimplementation manner of the fourth aspect of the present inventionincludes:

sending, by the UE, a CS call connection setup request message to amobile switching center MSC, where the CS call connection setup requestmessage is used to request the MSC to generate a session.

With reference to the fourth aspect of the present invention or thefirst implementation manner of the fourth aspect of the presentinvention, a second implementation manner of the fourth aspect of thepresent invention includes:

generating, by the UE, the session after receiving the CS callconnection setup request message sent by the MSC.

With reference to the fourth aspect of the present invention, the firstimplementation manner of the fourth aspect of the present invention, orthe second implementation manner of the fourth aspect of the presentinvention, a third implementation manner of the fourth aspect of thepresent invention includes: the handover command further includesinformation about a PS domain radio resource allocated by the 2G or 3Gnetwork to the UE for handover from the LTE network to a packet switchedPS domain of the 2G or 3G network.

A fifth aspect of the present invention provides a mobility managemententity MME, including:

a receiving unit, configured to receive a request message sent by userequipment UE located on a Long Term Evolution LTE network, where therequest message is used to request circuit switched fallback CSFB;

a generation unit, configured to generate a notification messageaccording to the request message received by the receiving unit, wherethe notification message is used to instruct an evolved NodeB eNodeB tomove the UE from the LTE network to a circuit switched CS domain of asecond generation 2G or third generation 3G network for the CSFB;

a sending unit, configured to send the notification message generated bythe generation unit to the eNodeB; and

a determining unit, configured to determine that an IMS voice session ofthe UE does not exist and determine not to delete a bearer correspondingto the IMS voice session, where

the receiving unit is further configured to receive a handoverrequirement message sent by the eNodeB:

the generation unit is further configured to generate a first handoverrequest message according to the handover requirement message receivedby the receiving unit, where the first handover request message is usedto request an MSC to hand over the UE from the LTE network to the CSdomain of the 2G or 3G network for the CSFB; and

the sending unit is further configured to: when the determining unitdetermines that the IMS voice session of the UE does not exist, send thefirst handover request message generated by the generation unit to themobile switching center MSC, so that the 2G or 3G network allocates a CSdomain resource to the UE, where the CS domain resource is used to handover the UE from the LTE network to the CS domain of the 2G or 3Gnetwork.

With reference to the fifth aspect of the present invention, a firstimplementation manner of the fifth aspect of the present inventionincludes:

the notification message is further used to indicate that the LTEnetwork, the 2G or 3G network, and the UE support the handover of the UEfrom the LTE network to the CS domain of the 2G or 3G network for theCSFB; or

the handover requirement message includes first indication information,where the first indication information is used to request the MME tohand over the UE from the LTE network to the CS domain of the 2G or 3Gnetwork for the CSFB; or

the first indication information is used to request the MME to hand overthe LTE from the LTE network to the CS domain of the 2G or 3G networkfor the CSFB and is used to indicate that the UE supports the handoverfrom the LTE network to the CS domain of the 2G or 3G network.

With reference to the first implementation manner of the fifth aspect ofthe present invention, a second implementation manner of the fifthaspect of the present invention includes: the notification messageincludes indication information, where the indication information isused to indicate that the LTE network, the 2G or 3G network, and the UEsupport the handover of the UE from the LTE network to the CS domain ofthe 2G or 3G network for the CSFB.

With reference to the second implementation manner of the fifth aspectof the present invention, a third implementation manner of the fifthaspect of the present invention includes:

the indication information is used to indicate that the LTE network, the2G or 3G network, and the UE support single radio voice call continuitySRVCC handover.

With reference to the fifth aspect of the present invention, the firstimplementation manner of the fifth aspect of the present invention, thesecond implementation manner of the fifth aspect of the presentinvention, or the third implementation manner of the fifth aspect of thepresent invention, a fourth implementation manner of the fifth aspect ofthe present invention includes:

the determining unit is further configured to determine that the LTEnetwork and the 2G or 3G network support the handover of the UE from theLTE network to the CS domain of the 2G or 3G network for the CSFB andthe UE supports the handover from the LTE network to the CS domain ofthe 2G or 3G network.

With reference to the fifth aspect of the present invention, the firstimplementation manner of the fifth aspect of the present invention, thesecond implementation manner of the fifth aspect of the presentinvention, the third implementation manner of the fifth aspect of thepresent invention, or the fourth implementation manner of the fifthaspect of the present invention, a fifth implementation manner of thefifth aspect of the present invention includes:

the receiving milt is further configured to receive a first handoverresponse message that is sent by the MSC to respond to the firsthandover request message, where the first handover response messageincludes information about a CS domain radio resource allocated by abase station on the 2G or 3G network to the UE;

the generation unit is further configured to generate a handoverrequirement response message according to the handover requirementmessage received by the receiving unit, where the handover requirementresponse message includes the information about the CS domain radioresource, and the information about the CS domain radio resource is usedto hand over the UE from the LTE network to the CS domain of the 2G or3G network for the CSFB; and

the sending unit is further configured to send the handover requirementresponse message generated by the generation unit to the eNodeB.

With reference to the fifth aspect of the present invention, the firstimplementation manner of the fifth aspect of the present invention, thesecond implementation manner of the fifth aspect of the presentinvention, the third implementation manner of the fifth aspect of thepresent invention, the fourth implementation manner of the fifth aspectof the present invention, or the fifth implementation manner of thefifth aspect of the present invention, a sixth implementation manner ofthe fifth aspect of the present invention includes:

the handover requirement message is a message for requiring handoverfrom the LTE network to a packet switched PS domain of the 2G or 3Gnetwork, or a message for requiring the handover from the LTE network tothe CS domain of the 2G or 3G network.

With reference to the fifth aspect of the present invention, the firstimplementation manner of the fifth aspect of the present invention, thesecond implementation manner of the fifth aspect of the presentinvention, the third implementation manner of the fifth aspect of thepresent invention, the fourth implementation manner of the fifth aspectof the present invention, or the fifth implementation manner of thefifth aspect of the present invention, a seventh implementation mannerof the fifth aspect of the present invention includes:

the handover requirement message is a message for requiring handoverfrom the LTE network to a PS domain and the CS domain of the 2G or 3Gnetwork.

With reference to the seventh implementation manner of the fifth aspectof the present invention, an eighth implementation manner of the fifthaspect of the present invention includes:

the sending unit is further configured to send a second handover requestmessage to a serving general packet radio service support node SGSN,where the second handover request message is used to request the SGSN tohand over the UE from the LTE network to the PS domain of the 2G or 3Gnetwork for the CSFB.

With reference to the fifth aspect of the present invention, the firstimplementation manner of the fifth aspect of the present invention, thesecond implementation manner of the fifth aspect of the presentinvention, the third implementation manner of the fifth aspect of thepresent invention, the fourth implementation manner of the fifth aspectof the present invention, the fifth implementation manner of the fifthaspect of the present invention, the sixth implementation manner of thefifth aspect of the present invention, the seventh implementation mannerof the fifth aspect of the present invention, or the eighthimplementation manner of the fifth aspect of the present invention, aninth implementation manner of the fifth aspect of the present inventionincludes:

the bearer corresponding to the IMS voice session is a bearer whose QoSclass identifier equals to 1.

A sixth aspect of the present invention provides an evolved NodeBeNodeB, including:

a receiving unit, configured to receive a notification message sent by amobility management entity MME, where the notification message is usedto instruct the eNodeB to move user equipment UE from a Long TermEvolution LTE network to a circuit switched CS domain of a 2G or 3Gnetwork for circuit switched fallback CSFB:

a generation unit, configured to generate a handover requirement messageaccording to the notification message received by the receiving unit;

a determining unit, configured to determine, according to a capabilityof the UE, that the UE supports the handover from the LTE network to theCS domain of the 2G or 3G network, where

the determining unit is further configured to determine that an IMSvoice session of the UE does not exist; and

a sending unit, configured to: when the determining unit determines thatthe IMS voice session of the UE does not exist, send the handoverrequirement message generated by the generation unit to the MME; where

the receiving unit is further configured to receive a handoverrequirement response message sent by the MME, where the handoverrequirement response message includes information about a CS domainradio resource, and the information about the C'S domain radio resourceis used to hand over the UE from the LTE network to the CS domain of the2G or 3G network:

the generation unit is further configured to generate a handover commandaccording to the handover requirement response message received by thereceiving unit, where the handover command is used to instruct that theUE be handed over from the LTE network to the CS domain of the 2G or 3Gnetwork, and the handover command includes the information about the CSdomain radio resource: and

the sending unit is further configured to send the handover commandgenerated by the generation unit to the UE.

With reference to the sixth aspect of the present invention, a firstimplementation manner of the sixth aspect of the present inventionincludes:

the notification message is further used to indicate that the LTEnetwork, the 2G or 3G network, and the UE support the handover of the UEfrom the LTE network to the CS domain of the 2G or 3G network for theCSFB; or

the handover command is used to instruct that the UE be handed over fromthe LTE network to the circuit switched CS domain of the 2G or 3Gnetwork for the CSFB.

With reference to the sixth aspect of the present invention or the firstimplementation manner of the sixth aspect of the present invention, asecond implementation manner of the sixth aspect of the presentinvention includes:

the determining unit is further configured to determine, by determiningthat a bearer whose QoS class identifier equals to 1 does not exist inthe UE, that the IMS voice session of the UE does not exist.

With reference to the sixth aspect of the present invention, the firstimplementation manner of the sixth aspect of the present invention, orthe second implementation manner of the sixth aspect of the presentinvention, a third implementation manner of the sixth aspect of thepresent invention includes:

the handover requirement message includes first indication information,where the first indication information is used to request the MME tohand over the UE from the LTE network to the CS domain of the 2G or 3Gnetwork for the CSFB; or

the first indication information is used to request the MME to hand overthe UE from the LTE network to the CS domain of the 2G or 3G network forthe CSFB and is used to indicate that the UE supports the handover fromthe LTE network to the CS domain of the 2G or 3G network; or

the handover requirement message includes a source-to-target transparentcontainer, where the source-to-target transparent container includessecond indication information, and the second indication information isused to request a base station on the 2G or 3G network to hand over theUE from the LTE network to the CS domain of the 2G or 3G network for theCSFB.

With reference to the third implementation maimer of the sixth aspect ofthe present invention, a fourth implementation manner of the sixthaspect of the present invention includes:

the second indication information is further used to indicate that theUE supports the handover from the LTE network to the CS domain of the 2Gor 3G network.

With reference to the sixth aspect of the present invention, the firstimplementation maimer of the sixth aspect of the present invention, thesecond implementation maimer of the sixth aspect of the presentinvention, the third implementation manner of the sixth aspect of thepresent invention, or the fourth implementation manner of the sixthaspect of the present invention, a fifth implementation manner of thesixth aspect of the present invention includes:

the handover requirement message is a message for requiring handoverfrom the LTE network to a packet switched PS domain of the 2G or 3Gnetwork, or a message for requiring the handover from the LTE network tothe CS domain of the 2G or 3G network.

With reference to the sixth aspect of the present invention, the firstimplementation manner of the sixth aspect of the present invention, thesecond implementation manner of the sixth aspect of the presentinvention, the third implementation manner of the sixth aspect of thepresent invention, or the fourth implementation manner of the sixthaspect of the present invention, a sixth implementation manner of thesixth aspect of the present invention includes:

the handover requirement message is a message for requiring handoverfrom the LTE network to a PS domain and the CS domain of the 2G or 3Gnetwork.

With reference to the sixth implementation manner of the sixth aspect ofthe present invention, a seventh implementation manner of the sixthaspect of the present invention includes:

the determining unit is further configured to determine that the UEsupports handover from the LTE network to the PS domain of the 2G or 3Gnetwork, where

the handover command is further used to instruct that the UE be handedover from the LTE network to the PS domain of the 2G or 3G network, thehandover command includes information about a PS domain resource of the2G or 3G network, and the information about the PS domain resource isused to hand over the UE from the LTE network to the PS domain of the 2Gor 3G network.

A seventh aspect of the present invention provides a mobile switchingcenter MSC, including:

a receiving unit, configured to receive a first handover request messagesent by a mobility management entity MME, where the first handoverrequest message is used to request the MSC to hand over user equipmentUE from a Long Term Evolution LTE network to a circuit switched CSdomain of a second generation 2G or third generation 3G network forcircuit switched fallback CSFB;

a generation unit, configured to generate a third handover requestmessage according to the first handover request message received by thereceiving unit, where the third handover request message is used torequest a base station on the 2G or 3G network to allocate a CS domainresource to the UE for the handover to the CS domain of the 2G or 3Gnetwork;

a determining unit, configured to determine, according to the firsthandover request message received by the receiving unit, not to initiatea session transfer process to an IP multimedia subsystem; and

a sending unit, configured to send the third handover request messagegenerated by the generation unit to the base station, where

the receiving unit is further configured to receive a third handoverrequest response message sent by the base station, where the thirdhandover request response message includes information about a CS domainradio resource allocated by the base station to the UE for the handoverto the CS domain of the 2G or 3G network;

the generation unit is further configured to generate a first handoverresponse message according to the first handover request messagereceived by the receiving unit, where the first handover responsemessage includes the information about the CS domain radio resource;

the sending unit is further configured to send the first handoverresponse message generated by the generation unit to the MME; and

the receiving unit is further configured to receive a handover completemessage sent by the UE, where the handover complete message indicatesthat the UE has already completed the handover from the LTE network tothe CS domain of the 2G or 3G network.

With reference to the seventh aspect of the present invention, a firstimplementation manner of the seventh aspect of the present inventionincludes: the first handover request message is further used to indicatethat the UE supports the handover from the LTE network to the CS domainof the 2G or 3G network.

With reference to the seventh aspect of the present invention or thefirst implementation manner of the seventh aspect of the presentinvention, a second implementation manner of the seventh aspect of thepresent invention includes:

the receiving unit is further configured to receive a first CS callconnection setup request message sent by the UE, where the first CS callconnection setup request message is used to request the MSC to generatea first session.

With reference to the seventh aspect of the present invention or thefirst implementation manner of the seventh aspect of the presentinvention, a third implementation manner of the seventh aspect of thepresent invention includes:

the sending unit is further configured to send a second CS callconnection setup request message to the UE, where the second CS callconnection setup request message is used to request the UE to generate asecond session.

With reference to the second implementation manner of the seventh aspectof the present invention, a fourth implementation manner of the seventhaspect of the present invention includes:

the sending unit is further configured to send a session release messageto the UE after the receiving unit receives the first CS call connectionsetup request message, where the session release message is used torequest the UE to release a third session generated by the UE in aprocess of the handover from the LTE network to the CS domain of the 2Gor 3G network.

With reference to the third implementation manner of the seventh aspectof the present invention, a fifth implementation manner of the seventhaspect of the present invention includes:

the receiving unit is further configured to receive a callacknowledgement message of the second CS call connection setup requestmessage; and

the sending unit is further configured to send a session release messageto the UE after the receiving unit receives the call acknowledgementmessage, where the session release message is used to request the UE torelease a third session generated by the UE in a process of the handoverfrom the LTE network to the CS domain of the 2G or 3G network.

An eighth aspect of the present invention provides user equipment UE,including:

a sending unit, configured to send a request message to a mobilitymanagement entity MME, where the request message is used to requestcircuit switched fallback CSFB;

a receiving unit, configured to receive a handover command sent by anevolved NodeB eNodeB, where the handover command is used to instructthat the UE be handed over from a Long Term Evolution LTE network to acircuit switched CS domain of a 2G or 3G network for the CSFB, and thehandover command includes information about a CS domain radio resourceallocated by the 2G or 3G network to the UE for the handover from theLTE network to the CS domain of the 2G or 3G network;

a handover unit, configured to hand over from the LTE network to the CSdomain of the 2G or 3G network according to the handover commandreceived by the receiving unit: and

a session generation unit, configured to generate a session after the UEis handed over from the LTE network to the CS domain of the 2G or 3Gnetwork.

With reference to the eighth aspect of the present invention, a firstimplementation manner of the eighth aspect of the present inventionincludes:

the sending unit is further configured to send a CS call connectionsetup request message to a mobile switching center MSC, where the CScall connection setup request message is used to request the MSC togenerate a session.

With reference to the eighth aspect of the present invention or thefirst implementation manner of the eighth aspect of the presentinvention, a second implementation manner of the eighth aspect of thepresent invention includes:

the session generation unit is configured to generate the session afterthe receiving unit receives the CS call connection setup request messagesent by the MSC.

A ninth aspect of the present invention provides a network handoversystem, including a mobility management entity, an evolved NodeB, amobile switching center, and user equipment, where

the mobility management entity is the mobility management entitydescribed in the foregoing fifth aspect;

the evolved NodeB is the evolved NodeB described in the foregoing sixthaspect;

the mobile switching center is the mobile switching center described inthe foregoing seventh aspect; and

the user equipment is the user equipment described in the foregoingeighth aspect.

In the embodiments provided in the present invention, in a process ofimplementing circuit switched fallback, handover from an LTE network toa 2G or 3G network is triggered, and UE can be handed over directly fromthe LTE network to a CS domain of the 2G or 3G network. Therefore, aresource is allocated on a network side to the UE for the handover fromthe LTE network to the CS domain of the 2G or 3G network, which avoidsallocation of a CS domain resource for the CSFB after the UE is handedover from the LTE network to a PS domain of the 2G or 3G network or isredirected to the 2G or 3G network, reduces an access delay of accessingthe CS domain of the 2G or 3G network by the UE, shortens entire voicecall duration, and improves user experience.

BRIEF DESCRIPTION OF DRAWINGS

To describe the technical solutions in the embodiments of the presentinvention more clearly, the following briefly describes the accompanyingdrawings required for describing the embodiments. Apparently, theaccompanying drawings in the following description show merely someembodiments of the present invention, and persons skilled in the art maystill derive other drawings from these accompanying drawings withoutcreative efforts.

FIG. 1 is a schematic diagram of an embodiment of a network handovermethod according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of another embodiment of a networkhandover method according to an embodiment of the present invention;

FIG. 3 is a schematic diagram of another embodiment of a networkhandover method according to an embodiment of the present invention;

FIG. 4 is a schematic diagram of another embodiment of a networkhandover method according to an embodiment of the present invention;

FIG. 5 is a schematic diagram of another embodiment of a networkhandover method according to an embodiment of the present invention;

FIG. 6 is a schematic diagram of another embodiment of a networkhandover method according to an embodiment of the present invention;

FIG. 7 is a schematic diagram of an embodiment of a mobility managemententity according to an embodiment of the present invention;

FIG. 8 is a schematic diagram of an embodiment of an evolved NodeBaccording to an embodiment of the present invention;

FIG. 9 is a schematic diagram of an embodiment of a mobile switchingcenter according to an embodiment of the present invention;

FIG. 10 is a schematic diagram of an embodiment of user equipmentaccording to an embodiment of the present invention; and

FIG. 11 is a schematic diagram of an embodiment of a network systemaccording to an embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention provide a network handover method,and in a process of implementing circuit switched fallback, handover ofUE from an LTE network to a CS domain of a 2G or 3G network istriggered, thereby reducing an access delay of accessing the CS domainof the 2G or 3G network by the UE, and also improving user experience.The embodiments of the present invention further provide a correspondingdevice and system. Detailed descriptions are separately given below.

Acronyms of some communications terms that are involved in theembodiments of the present invention include: mobility management entity(MME), evolved NodeB (eNodeB), user equipment (UE), mobile switchingcenter (MSC), the 2^(nd) generation or 3^(rd) generation of mobiletelecommunications technology (2rd Generation/3rd Generation, 2G or 3G),and base station (BS), Long Term Evolution (LTE), serving GPRS supportnode (SGSN), circuit switched fallback (CSFB), universal terrestrialradio access network (UTRAN), Global System for Mobile Communications(GSM), Enhanced Data rates for GSM Evolution (EDGE), and Global Systemfor Mobile Communications/Enhanced Data rates for GSM Evolution radioaccess network (GSM/EDGE Radio Access Network, GERAN), where a basestation in the embodiments of the present invention includes a basestation controller (BSC) and a radio network controller (RNC).

“First”, “second”, “third”, “fourth”, “fifth”, and the like, in theembodiments of the present invention are only used for distinguishingbetween indication information, messages, or other objects instead ofrepresenting a sequence relationship.

The following clearly describes the technical solutions in theembodiments of the present invention with reference to the accompanyingdrawings in the embodiments of the present invention. Apparently, thedescribed embodiments are merely some but not all of the embodiments ofthe present invention. All other embodiments obtained by persons skilledin the art based on the embodiments of the present invention withoutcreative efforts shall fall within the protection scope of the presentinvention.

Referring to FIG. 1, FIG. 1 shows a fallback process implemented byhanding UE over from an LTE network to a 2G or 3G network and an activecall process after the handover according to an embodiment of thepresent invention.

S100. The UE sends an extended service request to an MME, where theextended service request is used to request circuit switched fallbackCSFB.

S100 a. The MME may determine whether the LTE network and the 2G/3Gnetwork support handover of the UE from the LTE network to a CS domainof the 2G or 3G network for the CSFB.

Alternatively, the MME may not perform determining, but consider bydefault that communications networks support the handover of the UE fromthe LTE network to the CS domain of the 2G or 3G network for the CSFB,or may perform determining only on some of the networks.

The communications networks are the LTE network and the 2G or 3Gnetwork.

S105. The MME sends a notification message to an evolved NodeB eNodeBaccording to the extended service request, where the notificationmessage is used to instruct the eNodeB to move the UE from the LTEnetwork to the CS domain of the 2G or 3G network for the CSFB, that is,the notification message is used to instruct the enodeB that the UEshould be moved to the 2G or 3G network for the CSFB.

When the MME determines that the LTE network and the 2G or 3G networksupport the handover of the LTE from the LTE network to the CS domain ofthe 2G or 3G network for the CSFB, the MME may send the notificationmessage to inform the eNodeB whether the LTE network and the 2G or 3Gnetwork support the handover of the UE from the LTE network to the CSdomain of the 2G or 3G network for the CSFB, so that the eNodeBinitiates a handover requirement of handing over the UE from the LTEnetwork to the CS domain of the 2G or 3G network for the CSFB.

For example, the notification message may include indicationinformation, where the indication information is used to indicate thatthe communications networks and the UE support the handover of the LTEfrom the LTE network to the CS domain of the 2G or 3G network for theCSFB.

For example, the indication information may be indication information ofsingle radio voice call continuity (single radio voice call continuity,SRVCC) handover, and is used to indicate that the LTE network and the2G/3G network support handover of the UE from the LTE network to the 2Gor 3G network for the CSFB.

The indication information in this embodiment of the present inventionmay be referred to as an optimized CSFB indicator, a single radio voicecall continuity (SRVCC)-based CSFB indicator (SRVCC based CSFBIndicator), an enhanced CSFB indicator, an enhanced CSFB possibilityindicator, or an SRVCC operation possibility indicator.

S110. The eNodeB sends a handover requirement message to the MME.

It is determined, according to a capability of the UE, that the UEsupports the handover from the LTE network to the CS domain of the 2G or3G network. At the same time, the eNodeB determines that an IMS voicesession of the UE does not exist. The eNodeB sends the handoverrequirement message to the MME.

Specifically, the enodeB determines, according to an FGI (feature groupindicator) reported by the UE, whether the UE supports a handovercapability of being handed over from the LTE network to the CS domain ofthe 2G or 3G network. At the same time, the eNodeB determines that abearer whose QCI=1 does not exist, so as to determine that the IMS voicesession of the UE does not exist.

That is because hi the prior art, the enodeB can send the handoverrequirement message to the MME only when the LTE supports a capabilityof being handed over from the LTE network to the CS domain of the 2G or3G network and the enodeB also determines that the voice bearer whoseQCI=1 exists, so as to trigger the handover of the UE from the LTEnetwork to the CS domain of the 2G or 3G network.

The handover requirement message may be a message for requiring handoverfrom the LTE network to a packet switched PS domain of the 2G or 3Gnetwork (for example, an LTE to 2G/3G PS HO Required message), a messagefor requiring the handover from the LTE network to the CS domain of the2G or 3G network (for example, an LTE to 2G/3G SRVCC HO Requiredmessage), or a message for requiring handover from the LTE network tothe PS domain and the CS domain of the 2G or 3G network (for example, anLTE to 2G/3G SRVCC and PS HO Required message).

The handover requirement message may include UE capability informationthat is used to indicate information about a frequency band supported bythe UE, an access capability, and the like. For example, the UEcapability information may be at least one of a classmark 2 and aclassmark 3. In the prior art, when the UE does not support the handoverfrom the LTE network to the CS domain of the 2G/3G network, the eNodeBdoes not send the UE capability information to the MME, and therefore,the MME does not have the UE capability information. After the MMEacquires the UE capability information, the MME may send the UEcapability information to an MSC, so that the MSC can perform handoverto the CS domain of the 2G/3G. In this embodiment of the presentinvention, the UE capability information may be transferred to the MMEby using the handover requirement message.

The handover requirement message includes first indication information,where the first indication information is used to request the MME tohand over the UE from the LTE network to the CS domain of the 2G or 3Gnetwork for the CSFB; or

the first indication information is used to request the MME to hand overthe UE from the LTE network to the CS domain of the 2G or 3G network forthe CSFB and is used to indicate that the UE supports the handover fromthe LTE network to the CS domain of the 2G or 3G network; or

the handover requirement message includes a source-to-target transparentcontainer, where the source-to-target transparent container includessecond indication information, and the second indication information isused to request a base station on the 2G or 3G network to hand over theUE from the LTE network to the CS domain of the 2G or 3G network for theCSFB. The first indication information may be referred to as a CSFBindicator, an optimized CSFB indicator, an SRVCC-based CSFB indicator,an enhanced CSFB indicator, or an enhanced CSFB possibility indicator.

When the second indication information does not indicate that the UEsupports the handover from the LTE network to the CS domain of the 2G or3G network, the MME may consider by default that the UE supports thehandover. When the first indication information does not indicate thatthe UE supports the handover from the LTE network to the CS domain ofthe 2G or 3G network, the base station on the 2G/3G network may considerby default that the UE supports the handover. The second indicationinformation and the first indication information may be referred to as aCSFB indicator, an optimized CSFB indicator, an SRVCC-based CSFBindicator, an enhanced CSFB indicator, or an enhanced CSFB possibilityindicator.

For example, the second indication information may include a CSFBindication field, or a CSFB indication field and a UE capability field.The CSFB indication field instructs the MME to hand over the UE from theLTE network to the circuit switched CS domain of the 2G or 3G networkfor the CSFB. The UE capability field indicates that the UE supports thehandover from the LTE network to the PS domain of the 2G or 3G network.

The first indication information may include a CSFB indication field, ora CSFB indication field and a UE capability field. The CSFB indicationfield instructs the base station on the 2G/3G network to hand over theUE from the LTE network to the circuit switched CS domain of the 2G or3G network for the CSFB. The LTE capability field indicates that the UEsupports the handover from the LTE network to the PS domain of the 2G or3G network.

For example,

in a case in which a target network is a UTRAN network, the eNodeB addsthe second indication information to a source RNC to target RNCtransparent container, where a CSFB information field in the existingsource RNC to target RNC transparent container may be used as the CSFBindication field, and a field may be added to the source RNC to targetRNC transparent container and used as the UE capability field; and

in a case in which a target network is a GERAN network, the eNodeB addsthe second indication information to old BSS to new BSS information,where the CSFB indication field or the UE capability field may be addedto the old BSS to new BSS information.

S114. The MME determines that an IMS voice session of the UE does notexist and determines not to delete a bearer corresponding to the IMSvoice session.

Specifically, the MME determines that the bearer whose QCI=1 does notexist, so as to determine that the IMS voice session of the UE does notexist. And the MME still sends a first handover request message to theMSC.

Further, because the bearer whose QCI=1 does not exist, the MME does notneed to delete the bearer whose QCI=1.

That is because in the prior art, the MME can send the first handoverrequest message to the MSC and at the same time delete the voice bearerwhose QCI=1 only when an active IMS voice session currently exists, thatis, when the MME determines that the voice bearer whose QCI=1 exists.

S115. The MME sends a first handover request message to an MSC, wherethe first handover request message is used to request the MSC to handover the UE from the LTE network to the circuit switched CS domain ofthe second generation 2G or third generation 3G network for the CSFB.

Because the first handover request message is sent by the MME to theMSC, handover required in the first handover request message is thehandover from the LTE network to the CS domain of the 2G or 3G network,that is, the first handover request message is a PS-to-CS handoverrequest.

The first handover request message may carry third indicationinformation, where the third indication information is used to requestthe MSC to hand over the UE from the LTE network to the CS domain of the2G or 3G network for the CSFB, that is, the third indication informationis used to indicate that the first handover request message is sent forthe CSFB. The third indication information may further indicate that theUE supports the handover from the LTE network to the CS domain of the 2Gor 3G network. Alternatively, the first handover request message may notindicate the CSFB. In this case, the MSC considers by default that thefirst handover request message is sent for the CSFB.

After the MME receives the UE capability information from the eNodeB,the first handover request message may carry the UE capabilityinformation; for example, the UE capability information is at least oneof a classmark 2 and a classmark 3.

Because the UE does not support the handover from the LTE network to theCS domain of the 2G or 3G network, the UE does not report voicecoding/decoding information supported by the UE. In this case, the MMEmay add default voice coding/decoding information to the first handoverrequest message according to a target system of the handover.

The third indication information in the first handover request messagemay be referred to as a CSFB indicator, an optimized CSFB indicator(optimized CSFB indicator), a single radio voice call continuity (singleradio voice call continuity, SRVCC)-based CSFB indicator (SRVCC basedCSFB Indicator), an enhanced CSFB indicator (enhanced CSFB indicator),or an enhanced CSFB possibility indicator (enhanced CSFB possible).

When the first handover request message carries a session transfernumber-single radio (Session Transfer Number-Single Radio, STN-SR), theMSC ignores the STN-SR number.

S120. The MME sends a second handover request message to an SGSN, wherethe second handover request message is used to request the SGSN toinitiate handover from the LTE network to a packet switched PS domain ofthe 2G or 3G network.

When being handed over from the LTE network to the CS domain, the UE maybe handed over to the PS domain at the same time, or may not be handedover to the PS domain at the same time. In this way, dependence of thissolution on a network may be reduced, that is, an operator is requiredto support only the handover from the LTE network to the CS domain ofthe 2G or 3G network, and there is no need to force the operator tosupport the handover from the LTE network to the PS domain of the 2G or3G network.

S121. The MSC sends a third handover request message to a base station,where the third handover request message is used to request the basestation to allocate a CS domain resource to the UE for the handover tothe CS domain of the 2G or 3G network.

The MSC determines, according to the first handover request message, notto initiate a session transfer process to an IP multimedia subsystem.Specifically, the MSC does not send an Invite message to an IMS system,so as to avoid triggering a session transfer process.

The third handover request message is further used to request the basestation to hand over the user equipment UE from the Long Term EvolutionLTE network to the circuit switched CS domain of the 2G or 3G networkfor the CSFB. The base station allocates the CS domain resource to theUE for the handover from the LTE network to the CS domain of the 2G or3G network.

The MSC acquires PLMN information of the LTE network in a process inwhich the UE performs combined registration or from the first handoverrequest message sent by the MME.

For example, the CS domain resource allocated by the base stationincludes a wired resource between the base station and the MSC and aradio resource between the base station and the UE.

For example, the third handover request message includes fifthindication information, where the fifth indication information is usedto request the base station to hand over the UE from the LTE network tothe CS domain for the CSFB; and/or

the fifth indication information is used to indicate that the UEsupports the handover from the LTE network to the CS domain of the 2G or3G network.

The third handover request message includes a source-to-targettransparent container, where the source-to-target transparent containerincludes the fifth indication information.

When the first handover request message does not include the voicecoding/decoding information supported by the UE, the MSC sets defaultvoice coding/decoding information according to a type of the 2G or 3Gnetwork, where

the default voice coding/decoding information may be included in thethird handover request message.

The third handover request message includes integrity protectioninformation and/or encryption information, so that the base stationsends a security mode command to the UE after receiving a handovercomplete message.

S125. The base station sends a third handover request response messageto the MSC, where the third handover request response message includesinformation about the CS domain radio resource, and the informationabout the CS domain radio resource is used to hand over the UE from theLTE network to the CS domain of the 2G or 3G network.

S130. The MSC sends a first handover response message to the MME, wherethe first handover response message includes the information about theCS domain radio resource.

S135 a. The MME sends a handover requirement response message to theeNodeB, where the handover requirement response message includes theinformation about the CS domain radio resource.

S135 b. The eNodeB sends a handover command to the UE, where thehandover command includes the information about the CS domain radioresource.

For example, the handover command includes fourth indicationinformation, where the fourth indication information is used to instructthat the UE be handed over from the LTE network to the CS domain of the2G or 3G network for the CSFB.

For example, the handover command further includes information about aPS domain resource, where the UE is handed over from the LTE network tothe PS domain of the 2G or 3G network according to the information aboutthe PS domain resource.

The handover command may be further used to instruct that the UE behanded over from the LTE network to the PS domain of the 2G or 3Gnetwork.

S140. The UE is handed over from the LTE network to the CS domain of the2G or 3G network according to the handover command.

The UE is handed over from the LTE network to the CS domain of the 2G or3G network for the CSFB by using the information about the resource thatis included in the handover command.

The UE generates a third session in a process of the handover from theLTE network to the CS domain of the 2G or 3G network. Alternatively, theUE may not generate a session in the process of the handover from theLTE network to the CS domain of the 2G or 3G network.

When the information about the resource includes a CS encryptionalgorithm identifier, the UE activates CS domain encryption according tothe CS encryption algorithm identifier in the process of the handoverfrom the LTE network to the 2G or 3G network.

S145. After the handover is complete, the UE sends a handover complete(HO Complete) message to the base station (BSC or RNC).

The UE does not generate a session in the process of the handover fromthe LTE network to the CS domain of the 2G or 3G network for the CSFB.

Alternatively, in the process of the handover from the LTE network tothe CS domain of the 2G or 3G network for the CSFB, the UE generates asession, and after the handover is complete, deletes the session.

Further, when the UE confirms that there is no IMS session currently,the session is deleted. Specifically, the process of the handover fromthe LTE network to the CS domain of the 2G or 3G network is merely forreserving a voice session resource for the CSFB. That is because if anIMS session exists currently, the IMS session needs to be converted intoa session generated in the process of the handover from the LTE networkto the CS domain of the 2G or 3G network.

After receiving the handover complete message, to respond to thehandover complete message, the base station needs to add a location areaidentity (LAI) to UTRAN mobility information sent to the UE, which, inthis embodiment, may cause the UE to initiate location area update(Location Area Update, LAU), and lead to a failure in setting up a CScall. To avoid the failure, the UTRAN mobility information sent by thebase station to the UE may not carry the location area identity.

When sending the UTRAN mobility information in a subsequent process, thebase station may send the LAI to the UE by using the following exemplarymethod:

(a) after receiving the handover complete message, the base stationstarts a timer, and when the timer expires, the base station sends theUTRAN mobility information, so as to send the LAI to the UE; or

(b) after receiving a call IU release command (IU RELEASE COMMAND) or acall connection success (Connect) message, the base station sends theUTRAN mobility information, so as to send the LAI to the UE, where foran active call, the MSC sends a call connection success (Connect)message to the UE, and for a passive call, the UE sends a callconnection success (Connect) message to the MSC.

Alternatively, when the UE sends an LAU message to the base station, toavoid a failure, the base station does not send the location area updaterequest message to the MSC, but the base station sends a location areaupdate accept message directly to the UE.

S150. The base station forwards the handover complete message in stepS145 to the MSC.

S155. The UE sends a connection management service request (CM ServiceRequest) to the MSC.

S160. The MSC sends an authentication request (Authenticate Request) tothe UE.

Alternatively, this step S160 may not be executed, that is, the MSC doesnot send the authentication request message to the UE.

S165. The UE sends an authentication response (Authenticate Response) tothe MSC.

S170. The MSC sends a security mode command to the base station, wherethe security mode command carries integrity protection informationand/or encryption information (Encryption Information), so that the basestation sends the security mode command to the UE.

Alternatively, this step S170 may not be executed, that is, the MSC doesnot send the security mode command to the base station.

S175. The base station sends the security mode command to the UE.Specifically, the base station executes this step according to thesecurity mode command in step S170, or the base station executes thisstep after receiving the message in S145.

S180. Optionally, the MSC sends a connection management service acceptcommand (CM service Accept) to the UE.

S185. The UE sends a first CS call connection setup request message(Setup) to the MSC, where the first CS call connection setup requestmessage is used to request the MSC to generate a first session for theCSFB. Alternatively, the first CS call connection setup request messageis used to request the MSC to generate the third session for the CSFB.

S195. The MSC performs call peer end processing.

S196. The MSC sends an alerting (Alerting) message to the UE. The UEwaits for an answer message from the peer end.

Further, a called party picks up a phone to enter a conversation state.

After a call ends, because the call is triggered by the CSFB, the UEneeds to return to the LTE network after the call is complete. Accordingto the third handover request message, the base station determines thatthe call is triggered by the CSFB. Therefore, after the base station onthe 2G/3G network receives a call release message sent by the MSC, thebase station on the 2G/3G network redirects the UE to the LTE network orthe base station hands over the UE to the LTE network, where the callrelease message is a release command Iu Release Command or a clearcommand Clear Command.

It should be noted that in this embodiment, the base station can parseCS signaling between the UE and the MSC. The base station receives asignaling message sent by the UE to the MSC, where for example, theconnection management service request, the authentication response, thefirst CS call connection setup request message, and the like. The basestation may send the signaling message to the MSC through a connectionthat is set up in the process of the handover from the LTE network tothe CS domain of the 2G or 3G network, or send the signaling message tothe MSC by using a direct transfer message.

In this embodiment, the indication information included in thenotification message, the first indication information, the secondindication information, the third indication information, the fourthindication information, and the fifth indication information may includea CSFB indication field, or a CSFB indication field and a UE capabilityfield. The CSFB indication field instructs the MME, the MSC, or the basestation on the 2G/3G network to hand over the UE from the LTE network tothe circuit switched CS domain of the 2G or 3G network for the CSFB. TheUE capability field indicates that the UE supports the handover from theLTE network to the PS domain of the 2G or 3G network.

In FIG. 1, in a process of implementing circuit switched fallback,handover from an LTE network to a 2G or 3G network is triggered. Eventhough an IMS voice session does not exist on the networks, UE can behanded over directly from the LTE network to a CS domain of the 2G or 3Gnetwork. Therefore, a resource is allocated on a network side to the UEfor the handover from the LTE network to the CS domain of the 2G or 3Gnetwork, which avoids allocation of a CS domain resource for the CSFBafter the UE is handed over from the LTE network to a PS domain of the2G or 3G network or is redirected to the 2G or 3G network, and reducesan access delay of accessing the CS domain of the 2G or 3G network bythe UE. In addition, further, by determining that an IMS voice sessiondoes not exist, it is identified that the handover of the UE from theLTE network to the CS domain of the 2G or 3G network is triggered by theCSFB, so as to ensure that the handover from the LTE network to the CSdomain of the 2G or 3G network is properly performed.

Referring to FIG. 2, FIG. 2 shows a network handover process and apassive call process after the network handover according to anembodiment of the present invention.

S0 b and S0 c: An MSC sends a CS paging request to an MME, and the MMEtriggers a paging process.

Steps S200 to S250 are the same as steps S100 to S150 in FIG. 1, anddetails are not described herein again.

S255. The UE sends a paging response to the MSC.

S260. The MSC sends an authentication request to the UE.

Alternatively, this step S260 may not be executed, that is, the MSC doesnot send the authentication request message to the UE.

S265. The UE sends an authentication response to the MSC.

S270. The MSC sends a security mode command to the base station, wherethe security mode command carries integrity protection information(Integrity Protection Information) and/or encryption information(Encryption Information), so that the base station sends the securitymode command to the UE.

Alternatively, this step S270 may not be executed, that is, the MSC doesnot send the security mode command to the base station.

S275. The base station sends the security mode command to the UE.Specifically, the base station executes this step according to thesecurity mode command in step S270, or the base station executes thisstep after receiving the message in S245.

S280. The MSC sends a second CS call connection setup request message tothe UE, where the second CS call connection setup request message isused to request the LTE to generate a second session for the CSFB.

After a step of receiving, by the UE, the second CS call connectionsetup request message sent by the mobile switching center MSC, themethod further includes:

receiving or sending, by the UE, session data of the second session byusing the resource.

S285. The MSC receives a call acknowledgement message sent by the UE.

S295. The MSC sends an alerting (Alerting) message to the UE. The UEenters a conversation state.

S296. The MSC triggers alerting at a peer end.

After a call ends, because the call is triggered by the CSFB, the UEneeds to return to the LTE network after the call is complete. Accordingto the third handover request message, the base station determines thatthe call is triggered by the CSFB. Therefore, after the base station onthe 2G/3G network receives a call release message sent by the MSC, thebase station on the 2G/3G network redirects the UE to the LTE network orthe base station hands over the UE to the LTE network, where the callrelease message is a release command hi Release Command or a clearcommand Clear Command.

It should be noted that in this embodiment, the base station can parseCS signaling between the UE and the MSC. The base station receives asignaling message sent by the UE to the MSC, where for example, the callacknowledgement message, the authentication response, the second CS callconnection setup request message, and the like. The base station maysend the signaling message to the MSC through a connection that is setup in the process of the handover from the LTE network to the CS domainof the 2G or 3G network, or send the signaling message to the MSC byusing a direct transfer message.

In this embodiment, the indication information included in thenotification message, the second indication information, the thirdindication information, the fourth indication information, and the fifthindication information may include a CSFB indication field, or a CSFBindication field and a UE capability field. The CSFB indication fieldinstructs the MME to hand over the UE from the LTE network to thecircuit switched CS domain of the 2G or 3G network for the CSFB. The UEcapability field indicates that the UE supports the handover from theLTE network to the PS domain of the 2G or 3G network.

In this embodiment provided in the present invention, in a process ofimplementing circuit switched fallback, handover from an LTE network toa 2G or 3G network is triggered, and UE can be handed over directly fromthe LTE network to a CS domain of the 2G or 3G network. Therefore, aresource is allocated on a network side to the UE for the handover fromthe LTE network to the CS domain of the 2G or 3G network, which avoidsallocation of a CS domain resource for the CSFB after the UE is handedover from the LTE network to a PS domain of the 2G or 3G network or isredirected to the 2G or 3G network, reduces an access delay of accessingthe CS domain of the 2G or 3G network by the UE, shortens entire voicecall duration, and improves user experience. In addition, further, bydetermining that an IMS voice session does not exist, it is identifiedthat the handover of the UE from the LTE network to the CS domain of the2G or 3G network is triggered by the CSFB, so as to ensure that handoverfrom the LTE network is properly performed.

In the foregoing two application scenarios, an entire network handoverand session process is described from two separate perspectives: the UEas a caller and the UE as a callee.

Referring to FIG. 3, an embodiment of a network handover method providedin an embodiment of the present invention includes the following steps:

101. A mobility management entity MME receives a request message sent byuser equipment UE located on a Long Term Evolution LTE network, wherethe request message is used to request circuit switched fallback CSFB.

The request message may be an extended service request message.

102. The MME sends a notification message to an evolved NodeB eNodeBaccording to the request message, where the notification message is usedto instruct the eNodeB to move the UE from the LTE network to a circuitswitched CS domain of a 2G or 3G network for the CSFB.

103. The MME receives a handover requirement message sent by the eNodeB.

104. The MME determines that an IMS voice session of the UE does notexist and determines not to delete a bearer corresponding to the IMSvoice session.

105. The MME sends a first handover request message to a mobileswitching center MSC, where the first handover request message is usedto request the MSC to hand over the UE from the LTE network to the CSdomain of the 2G or 3G network for the CSFB, so that the 2G or 3Gnetwork allocates a CS domain resource to the UE, and the CS domainresource is used to hand over the UE from the LTE network to the CSdomain of the 2G or 3G network.

For the detailed description of the foregoing processing steps andanother processing step of the MME in this embodiment, reference may bemade to the description in the embodiment corresponding to FIG. 1 orFIG. 2, and details are not described herein again.

Referring to FIG. 4, another embodiment of a network handover methodprovided in an embodiment of the present invention includes thefollowing steps:

201. An evolved NodeB eNodeB located on a Long Term Evolution LTEnetwork receives a notification message sent by a mobility managemententity MME, where the notification message is used to instruct theeNodeB to move user equipment from the LTE network to a circuit switchedCS domain of a 2G or 3G network for circuit switched fallback CSFB.

202. Determine, according to a capability of the UE, that the UEsupports the handover from the LTE network to the CS domain of the 2G or3G network.

203. The eNodeB determines that an IMS voice session of the UE does notexist, and the eNodeB sends a handover requirement message to the MME.

204. The eNodeB receives a handover requirement response message sent bythe MME, where the handover requirement response message includesinformation about a CS domain radio resource, and the information aboutthe CS domain radio resource is used to hand over the UE from the LTEnetwork to the CS domain of the 2G or 3G network.

205. The eNodeB sends a handover command to the UE, where the handovercommand is used to instruct that the UE be handed over from the LTEnetwork to the CS domain of the 2G or 3G network, and the handovercommand includes the information about the CS domain radio resource.

For the detailed description of the foregoing processing steps andanother processing step of the evolved NodeB in this embodiment,reference may be made to the description in the embodiment correspondingto FIG. 1 or FIG. 2, and details are not described herein again.

Referring to FIG. 5, another embodiment of a network handover methodprovided in an embodiment of the present invention includes thefollowing steps:

301. A mobile switching center MSC receives a first handover requestmessage sent by a mobility management entity MME, where the firsthandover request message is used to request the MSC to hand over userequipment UE from an LTE network to a circuit switched CS domain of asecond generation 2G or third generation 3G network for circuit switchedfallback CSFB.

302. The MSC determines, according to the first handover requestmessage, not to initiate a session transfer process to an IP multimediasubsystem.

303. The MSC sends a third handover request message to a base station onthe 2G or 3G network, where the third handover request message is usedto request the base station to allocate a CS domain resource to the UEfor the handover to the CS domain of the 2G or 3G network.

304. The MSC receives a third handover request response message sent bythe base station, where the third handover request response messageincludes information about a CS domain radio resource allocated by thebase station to the UE for the handover to the CS domain of the 2G or 3Gnetwork.

305. The MSC sends a first handover response message to the MME, wherethe first handover response message includes the information about theCS domain radio resource.

306. The MSC receives a handover complete message sent by the UE, wherethe handover complete message indicates that the UE has alreadycompleted the handover from the LTE network to the CS domain of the 2Gor 3G network.

The method in this embodiment may further include the following step:

receiving, by the MSC, a first CS call connection setup request messagesent by the UE, where the first CS call connection setup request messageis used to request the MSC to generate a first session for the CSFB.This step is an active call process of the UE.

The method in this embodiment may further include the following step:sending, by the MSC, a second CS call connection setup request messageto the UE, where the second CS call connection setup request message isused to request the UE to generate a second session for the CSFB. Thisstep is a passive call process of the UE.

For the detailed description of the foregoing processing steps andanother processing step of the UE in this embodiment, reference may bemade to the description in the embodiment corresponding to FIG. 1 orFIG. 2, and details are not described herein again.

Referring to FIG. 6, another embodiment of a network handover methodprovided in an embodiment of the present invention includes thefollowing steps:

401. User equipment UE located on a Long Term Evolution LTE networksends a request message to a mobility management entity MME, where therequest message is used to request circuit switched fallback CSFB.

The request message may be an extended service request.

402. The UE receives a handover command sent by an evolved NodeB eNodeB,where the handover command is used to instruct that the UE be handedover from the LTE network to a circuit switched CS domain of a 2G or 3Gnetwork for the CSFB, and the handover command includes informationabout a CS domain radio resource allocated by the 2G or 3G network tothe UE for the handover from the LTE network to the circuit switched CSdomain of the 2G or 3G network.

403. Hand over the UE from the LTE network to the CS domain of the 2G or3G network according to the handover command.

404. The UE generates a session after the UE is handed over from the LTEnetwork to the CS domain of the 2G or 3G network.

The method shown in FIG. 6 may further include:

sending, by the UE in the CS domain, a first CS call connection setuprequest message to a mobile switching center MSC, where the first CScall connection setup request message is used to request the MSC togenerate a first session for the CSFB. This step is an active callprocess.

The method shown in FIG. 6 may further include: receiving, by the UE inthe CS domain, a second CS call connection setup request message sent bythe MSC, where the second CS call connection setup request message isused to request the UE to generate a second session for the CSFB. Thisstep is a passive call process.

For the detailed description of the foregoing processing steps andanother processing step of the UE in this embodiment, reference may bemade to the description in the embodiment corresponding to FIG. 1 orFIG. 2, and details are not described herein again.

Referring to FIG. 7, a mobility management entity MIME 60 provided in anembodiment of the present invention includes: a receiving unit 601, ageneration unit 602, a sending unit 603, and a determining unit 604.

The receiving unit 601 is configured to receive a request message sentby user equipment UE located on a Long Term Evolution LTE network, wherethe request message is used to request circuit switched fallback CSFB.

The generation unit 602 is configured to generate a notification messageaccording to the request message received by the receiving unit 601,where the notification message is used to instruct an evolved NodeBeNodeB to move the UE from the LTE network to a circuit switched CSdomain of a second generation 2G or third generation 3G network for theCSFB.

The sending unit 603 is configured to send the notification messagegenerated by the generation unit 602 to the eNodeB.

The determining unit 604 is configured to determine that an IMS voicesession of the UE does not exist and determine not to delete a bearercorresponding to the IMS voice session.

The receiving unit 601 is further configured to receive a handoverrequirement message sent by the eNodeB.

The generation unit 602 is further configured to generate a firsthandover request message according to the handover requirement messagereceived by the receiving unit 601, where the first handover requestmessage is used to request an MSC to hand over the UE from the LTEnetwork to the CS domain of the 2G or 3G network for the CSFB.

The sending unit 603 is further configured to: when the determining unitdetermines that the IMS voice session of the UE does not exist, send thefirst handover request message generated by the generation unit to themobile switching center MSC, so that the 2G or 3G network allocates a CSdomain resource to the UE, where the CS domain resource is used to handover the UE from the LTE network to the CS domain of the 2G or 3Gnetwork.

The determining unit 604 is further configured to determine that the LTEnetwork and the 2G or 3G network support the handover of the UE from theLTE network to the CS domain of the 2G or 3G network for the CSFB andthe UE supports the handover from the LTE network to the CS domain ofthe 2G or 3G network.

The receiving unit 601 is further configured to receive a first handoverresponse message that is sent by the MSC to respond to the firsthandover request message, where the first handover response messageincludes information about a CS domain radio resource allocated by abase station on the 2G or 3G network to the UE.

The generation unit 602 is further configured to generate a handoverrequirement response message according to the handover requirementmessage received by the receiving unit 601, where the handoverrequirement response message includes the information about the CSdomain radio resource, and the information about the CS domain radioresource is used to hand over the UE from the LTE network to the CSdomain of the 2G or 3G network for the CSFB.

The sending unit 603 is further configured to send the handoverrequirement response message generated by the generation unit 602 to theeNodeB.

The mobility management entity in this embodiment can implement themethod steps of the MME described in the method embodiments in FIG. 1,FIG. 2, and FIG. 3. The receiving unit 601 is configured to receive amessage sent by another entity to the MME, and the sending unit 603 isconfigured to send a message to another entity. Persons skilled in theart may understand, according to the steps executed in the methodembodiments, an action of and an action executed by each unit and arelationship between the units in the MME.

Referring to FIG. 8, all evolved NodeB 70 provided in all embodiment ofthe present invention includes: a receiving unit 701, a generation unit702, a determining unit 703, and a sending unit 704.

The receiving unit 701 is configured to receive a notification messagesent by a mobility management entity MME, where the notification messageis used to instruct the eNodeB to move user equipment UE from a LongTerm Evolution LTE network to a circuit switched CS domain of a 2G or 3Gnetwork for circuit switched fallback CSFB.

The generation unit 702 is configured to generate a handover requirementmessage according to the notification message received by the receivingunit 701.

The determining unit 703 is configured to determine, according to acapability of the UE, that the UE supports the handover from the LTEnetwork to the CS domain of the 2G or 3G network.

The determining unit 703 is further configured to determine that an IMSvoice call of the UE does not exist.

The sending unit 704 is configured to: when the determining unit 703determines that the IMS voice session of the UE does not exist, send thehandover requirement message generated by the generation unit 702 to theMME.

The receiving unit 701 is further configured to receive a handoverrequirement response message sent by the MME, where the handoverrequirement response message includes information about a CS domainradio resource, and the information about the CS domain radio resourceis used to hand over the UE from the LTE network to the CS domain of the2G or 3G network.

The generation unit 702 is further configured to generate a handovercommand according to the handover requirement response message receivedby the receiving unit 701, where the handover command is used toinstruct that the UE be handed over from the LTE network to the CSdomain of the 2G or 3G network, and the handover command includes theinformation about the CS domain radio resource.

The sending unit 704 is further configured to send the handover commandgenerated by the generation unit 702 to the UE.

The determining unit 703 is further configured to determine, bydetermining that a bearer whose QoS class identifier equals to 1 doesnot exist in the UE, that the IMS voice session of the UE does notexist.

The determining unit 703 is further configured to determine that the UEsupports handover from the LTE network to a PS domain of the 2G or 3Gnetwork, where

the handover command is further used to instruct that the UE be handedover from the LTE network to the PS domain of the 2G or 3G network, thehandover command includes information about a PS domain resource of the2G or 3G network,

and the information about the PS domain resource is used to hand overthe UE from the LTE network to the PS domain of the 2G or 3G network.

When a handover requirement is a requirement of handing over from theLTE network to the CS domain and the PS domain of the 2G or 3G network,the handover command is further used to instruct that the UE be handedover from the LTE network to the PS domain of the 2G or 3G network, thehandover command includes the information about the PS domain resourceof the 2G or 3G network, and the information about the PS domainresource is used to hand over the UE from the LTE network to the PSdomain of the 2G or 3G network.

The evolved NodeB in this embodiment can implement the method steps ofthe evolved NodeB described in the method embodiments in FIG. 1, FIG. 2,and FIG. 4. The receiving unit 701 is configured to receive a messagesent by another entity to the evolved NodeB, and the sending unit 704 isconfigured to send a message to another entity. Persons skilled in theart may understand, according to the steps executed in the methodembodiments, an action of and an action executed by each unit and arelationship between the units in the evolved NodeB.

Referring to FIG. 9, a mobile switching center MSC 80 provided in anembodiment of the present invention includes: a receiving unit 801, ageneration unit 802, a determining unit 803, and a sending unit 804.

The receiving unit 801 is configured to receive a first handover requestmessage sent by a mobility management entity MME, where the firsthandover request message is used to request the MSC to hand over userequipment UE from an LTE network to a circuit switched CS domain of asecond generation 2G or third generation 3G network for circuit switchedfallback CSFB.

The generation unit 802 is configured to generate a third handoverrequest message according to the first handover request message receivedby the receiving unit 801, where the third handover request message isused to request a base station on the 2G or 3G network to allocate a CSdomain resource to the UE for the handover to the CS domain of the 2G or3G network.

The determining unit 803 is configured to determine, according to thefirst handover request message received by the receiving unit 801, notto initiate a session transfer process to an IP multimedia subsystem.

The sending unit 804 is configured to send the third handover requestmessage generated by the generation unit 802 to the base station.

The receiving unit 801 is further configured to receive a third handoverrequest response message sent by the base station, where the thirdhandover request response message includes information about a CS domainradio resource allocated by the base station to the UE for the handoverto the CS domain of the 2G or 3G network.

The generation unit 802 is further configured to generate a firsthandover response message according to the first handover requestmessage received by the receiving unit 801, where the first handoverresponse message includes the information about the CS domain radioresource.

The sending unit 804 is further configured to send the first handoverresponse message generated by the generation unit 802 to the MME.

The receiving unit 801 is further configured to receive a handovercomplete message sent by the UE, where the handover complete messageindicates that the UE has already completed the handover from the LTEnetwork to the CS domain of the 2G or 3G network.

The receiving unit 801 is further configured to receive a first CS callconnection setup request message sent by the UE, where the first CS callconnection setup request message is used to request the MSC to generatea first session.

The sending unit 804 is further configured to send a second CS callconnection setup request message to the UE, where the second CS callconnection setup request message is used to request the UE to generate asecond session.

The sending unit 804 is further configured to send a session releasemessage to the UE after the receiving unit receives the first CS callconnection setup request message, where the session release message isused to request the UE to release a third session generated by the UE ina process of the handover from the LTE network to the CS domain of the2G or 3G network.

The sending unit 804 is further configured to send a session releasemessage to the UE after the receiving unit receives a callacknowledgement message, where the session release message is used torequest the UE to release a third session generated by the UE in theprocess of the handover from the LTE network to the CS domain of the 2Gor 3G network.

The MSC in this embodiment can implement the method steps of the MSCdescribed in the method embodiments in FIG. 1, FIG. 2, and FIG. 5. Thereceiving unit 801 is configured to receive a message sent by anotherentity to the MSC, and the sending unit 804 is configured to send amessage to another entity. Persons skilled in the art may understand,according to the steps executed in the method embodiments, an action ofand an action executed by each unit and a relationship between the unitsin the MSC.

Referring to FIG. 10, user equipment 90 provided in an embodiment of thepresent invention includes: a sending unit 901, a receiving unit 902, ahandover unit 903, and a session generation unit 904.

The sending unit 901 is configured to send a request message to amobility management entity MME, where the request message is used torequest circuit switched fallback CSFB.

The receiving unit 902 is configured to receive a handover command sentby an evolved NodeB eNodeB, where the handover command is used toinstruct that the UE be handed over from a Long Term Evolution LTEnetwork to a circuit switched CS domain of a 2G or 3G network for theCSFB, and the handover command includes information about a CS domainradio resource allocated by the 2G or 3G network to the UE for thehandover from the LTE network to the CS domain of the 2G or 3G network.

The handover unit 903 is configured to hand over from the LTE, networkto the CS domain of the 2G or 3G network according to the informationabout the resource that is received by the receiving unit 902.

The session generation unit 904 is configured to generate a sessionafter the handover unit 903 hands over from the LTE network to the CSdomain of the 2G or 3G network.

The sending unit 901 is further configured to send a CS call connectionsetup request message to a mobile switching center MSC, where the CScall connection setup request message is used to request the MSC togenerate a session.

The session generation unit 904 is configured to generate the sessionafter the receiving unit 902 receives the CS call connection setuprequest message sent by the MSC.

The UE in this embodiment can implement the method steps of the UEdescribed in the method embodiments in FIG. 1, FIG. 2, and FIG. 6. Thereceiving unit 902 is configured to receive a message sent by anotherentity to the UE, and the sending unit 901 is configured to send amessage to another entity. Persons skilled in the art may understand,according to the steps executed in the method embodiments, an action ofand an action executed by each unit and a relationship between the unitsin the UE.

Persons skilled in the art can understand that in the embodiments inFIG. 7 to FIG. 10, the sending unit used to send a message may beimplemented by a transmitter, and the receiving unit used to receive amessage may be implemented by a receiver. Alternatively, both thesending unit and the receiving unit may be implemented by a transceiver.In physical implementation, the transmitter or the transceiver may beimplemented by one physical entity or by multiple physical entities, andthe transmitter and the transceiver may be implemented by one physicalentity or by multiple physical entities, which is not limited in thepresent invention. Other units such as the generation unit, anallocation unit, a redirection unit, the handover unit, the determiningunit, and the like may be implemented by one or more processors, whichis not limited in the present invention.

Referring to FIG. 11, an embodiment of a network handover systemprovided in an embodiment of the present invention includes: a mobilitymanagement entity 60, an evolved NodeB 70, a mobile switching center 80,a base station 100, and user equipment 90.

For actions executed by and interactions between the mobility managemententity 60, the evolved NodeB 70, the mobile switching center 80, thebase station 100, and the user equipment 90, reference may be made tothe descriptions about FIG. 1 and FIG. 2 and descriptions about themethod embodiments and the apparatus embodiments in FIG. 3 to FIG. 10,and details are not described herein again.

Persons of ordinary skill in the art may be aware that, in combinationwith the examples described in the embodiments disclosed in thisspecification, units and algorithm steps may be implemented byelectronic hardware, computer software, or a combination thereof. Toclearly describe the interchangeability between the hardware and thesoftware, the foregoing has generally described compositions and stepsof each example according to functions. Whether the functions areperformed by hardware or software depends on particular applications anddesign constraint conditions of the technical solutions. Persons skilledin the art may use different methods to implement the describedfunctions for each particular application, but it should not beconsidered that the implementation goes beyond the scope of the presentinvention.

It may be clearly understood by persons skilled in the art that, for thepurpose of convenient and brief description, for a detailed workingprocess of the foregoing system, apparatus, and unit, reference may bemade to a corresponding process in the foregoing method embodiments, anddetails are not described herein again.

In the several embodiments provided in the present application, itshould be understood that the disclosed system, apparatus, and methodmay be implemented in other manners. For example, the describedapparatus embodiment is merely exemplary. For example, the unit divisionis merely logical function division and may be other division in actualimplementation. For example, a plurality of units or components may becombined or integrated into another system, or some features may beignored or not performed. In addition, the displayed or discussed mutualcouplings or direct couplings or communication connections may beimplemented through some interfaces. The indirect couplings orcommunication connections between the apparatuses or units may beimplemented in electronic, mechanical, or other forms.

The units described as separate parts may or may not be physicallyseparate, and parts displayed as units may or may not be physical units,may be located in one position, or may be distributed on a plurality ofnetwork units. A part or all of the units may be selected according toactual needs to achieve the objectives of the solutions of theembodiments of the present invention.

In addition, functional units in the embodiments of the presentinvention may be integrated into one processing unit, or each of theunits may exist alone physically, or two or more units are integratedinto one unit. The integrated unit may be implemented in a form ofhardware, or may be implemented in a form of a software functional unit.

When the integrated unit is implemented in the form of a softwarefunctional unit and sold or used as an independent product, theintegrated unit may be stored in a computer-readable storage medium.Based on such an understanding, the technical solutions of the presentinvention essentially, or the part contributing to the prior art, or allor a part of the technical solutions may be implemented in the form of asoftware product. The software product is stored in a storage medium andincludes several instructions for instructing a computer device (whichmay be a personal computer, a server, or a network device) to performall or a part of the steps of the methods described in the embodimentsof the present invention. The foregoing storage medium includes: anymedium that can store program code, such as a USB flash drive, aremovable hard disk, a read-only memory (ROM, Read-Only Memory), arandom access memory (RAM, Random Access Memory), a magnetic disk, or anoptical disc.

The LTE network mentioned in the present invention includes an LTE Anetwork and subsequent possible LTE versions.

The foregoing descriptions are merely specific embodiments of thepresent invention, but are not intended to limit the protection scope ofthe present invention. Any modification or replacement readily figuredout by persons skilled in the art within the technical scope disclosedin the present invention shall fall within the protection scope of thepresent invention. Therefore, the protection scope of the presentinvention shall be subject to the protection scope of the claims.

What is claimed is:
 1. A system, comprising: a mobility managemententity (MME), configured to receive a request message from userequipment (UE) located on a Long Term Evolution (LTE) network, whereinthe request message is used to request circuit switched fallback (CSFB);send a notification message to an evolved NodeB (eNodeB) according tothe request message, wherein the notification message is used toinstruct the eNodeB to move the UE from the LTE network to a circuitswitched (CS) domain of a second generation (2G) or third generation(3G) network for the CSFB; the eNodeB, configured to receive thenotification message, determine, according to a capability of the UE,that the UE supports the handover from the LTE network to the CS domainof the 2G or 3G network, determine that an IMS voice session of the UEdoes not exist, and send a handover requirement message to the MME; theMME, further configured to receive the handover requirement message fromthe eNodeB, determine that an IMS voice session of the UE does notexist, and send a first handover request message to a mobile switchingcenter (MSC), wherein the first handover request message is used torequest the MSC to hand over the UE from the LTE network to the CSdomain of the 2G or 3G network for the CSFB; and the MSC, configured toreceive the first handover request message from the MME, determine,according to the first handover request message, not to initiate asession transfer process to an IP multimedia subsystem, send a thirdhandover request message to a base station on the 2G or 3G network,wherein the third handover request message is used to request the basestation to allocate a CS domain resource to the UE for the handover tothe CS domain of the 2G or 3G network, receive a third handover requestresponse message from the base station, wherein the third handoverrequest response message comprises information about a CS domain radioresource allocated by the base station to the UE for the handover to theCS domain of the 2G or 3G network, and send a first handover responsemessage to the MME, wherein the first handover response messagecomprises the information about the CS domain radio resource.
 2. Thesystem according to claim 1, wherein: the MME is further configured toreceive the first handover response message from the MSC to respond tothe first handover request message; and send a handover requirementresponse message to the eNodeB, wherein the handover requirementresponse message comprises the information about the CS domain radioresource; the eNodeB is further configured to receive the handoverrequirement response message from the MME, and send a handover commandto the UE, wherein the handover command is used to instruct that the UEbe handed over from the LTE network to the CS domain of the 2G or 3Gnetwork, and the handover command comprises the information about the CSdomain radio resource.
 3. The system according to claim 2, wherein theUE is configured to receive the handover command from the eNodeB, handover the UE from the LTE network to the CS domain of the 2G or 3Gnetwork according to the handover command, generate a session after theUE is handed over from the LTE network to the CS domain of the 2G or 3Gnetwork.
 4. The system according to claim 3, wherein: the UE is furtherconfigured to send a handover complete message, wherein the handovercomplete message indicates that the UE has already completed thehandover from the LTE network to the CS domain of the 2G or 3G network;and the MSC, further configured to receive the handover complete messagefrom the UE.
 5. The system according to claim 1, wherein the eNodeB isfurther configured to determine that a bearer whose QoS class identifierequals to 1 does not exist in the UE, that the IMS voice session of theUE does not exist.
 6. The system according to claim 1, wherein the firsthandover request message is further used to indicate that the UEsupports the handover from the LTE network to the CS domain of the 2G or3G network.
 7. The system according to claim 1, wherein the MSC isfurther configured to receive a first CS call connection setup requestmessage from the UE, wherein the first CS call connection setup requestmessage is used to request the MSC to generate a first session.
 8. Thesystem according to claim 1, wherein the MSC is further configured tosend a second CS call connection setup request message to the UE,wherein the second CS call connection setup request message is used torequest the UE to generate a second session.
 9. A device, comprising: areceiver, configured to receive a request message from user equipment(UE) located on a Long Term Evolution (LTE) network, wherein the requestmessage is used to request circuit switched fallback (CSFB); aprocessor, configured to generate a notification message according tothe request message received by the receiver, wherein the notificationmessage is used to instruct an evolved NodeB (eNodeB) to move the UEfrom the LTE network to a circuit switched (CS) domain of a secondgeneration (2G) or third generation (3G) network for the CSFB; and atransmitter, configured to send the notification message to the eNodeB;and the processor is further configured to determine that an IMS voicesession of the UE does not exist and determine not to delete a bearercorresponding to the IMS voice session, wherein the receiver is furtherconfigured to receive a handover requirement message from the eNodeB;the processor is further configured to generate a first handover requestmessage according to the handover requirement message received by thereceiver, wherein the first handover request message is used to requesta mobile switching center (MSC) to hand over the UE from the LTE networkto the CS domain of the 2G or 3G network for the CSFB; and thetransmitter is further configured to: when the processor determines thatthe IMS voice session of the UE does not exist, send the first handoverrequest message generated by the processor to the MSC, so that the 2G or3G network allocates a CS domain radio resource to the UE, wherein theCS domain radio resource is used to hand over the UE from the LTEnetwork to the CS domain of the 2G or 3G network.
 10. The deviceaccording to claim 9, wherein: the receiver is further configured toreceive a first handover response message from the MSC to respond to thefirst handover request message, wherein the first handover responsecomprises information about the CS domain radio resource; and thetransmitter is further configured to send a handover requirementresponse message to the eNodeB, wherein the handover requirementresponse message comprises the information about the CS domain radioresource.
 11. A device, comprising: a receiver, configured to receive anotification message from a mobility management entity (MME), whereinthe notification message is used to instruct the eNodeB to move userequipment (UE) from a Long Term Evolution (LTE) network to a circuitswitched (CS) domain of a 2G or 3G network for circuit switched fallback(CSFB); a processor, configured to generate a handover requirementmessage according to the notification message received by the receiver;and determine, according to a capability of the UE, that the UE supportsthe handover from the LTE network to the CS domain of the 2G or 3Gnetwork, wherein the processor is further configured to determine thatan IMS voice session of the UE does not exist; and a transmitter,configured to: when the processor determines that the IMS voice sessionof the UE does not exist, send the handover requirement messagegenerated by the processor to the MME; wherein the receiver is furtherconfigured to receive a handover requirement response message from theMME, wherein the handover requirement response message comprisesinformation about a CS domain radio resource, and the information aboutthe CS domain radio resource is used to hand over the UE from the LTEnetwork to the CS domain of the 2G or 3G network; the processor isfurther configured to generate a handover command according to thehandover requirement response message received by the receiver, whereinthe handover command is used to instruct that the UE be handed over fromthe LTE network to the CS domain of the 2G or 3G network, and thehandover command comprises the information about the CS domain radioresource; and the transmitter is further configured to send the handovercommand generated by the processor to the UE.
 12. The device accordingto claim 11, wherein: the processor is further configured to determine,by determining that a bearer whose QoS class identifier equals to 1 doesnot exist in the UE, that the IMS voice session of the UE does notexist.
 13. A device, comprising: a receiver, configured to receive afirst handover request message from a mobility management entity (MME),wherein the first handover request message is used to request a mobileswitching center (MSC) to hand over user equipment (UE) from a Long TermEvolution (LTE) network to a circuit switched (CS) domain of a secondgeneration (2G) or third generation (3G) network for circuit switchedfallback (CSFB); a processor, configured to generate a third handoverrequest message according to the first handover request message receivedby the receiver, wherein the third handover request message is used torequest a base station on the 2G or 3G network to allocate a CS domainresource to the UE for the handover to the CS domain of the 2G or 3Gnetwork; and determine, according to the first handover request messagereceived by the receiver, not to initiate a session transfer process toan IP multimedia subsystem; and a transmitter, configured to send thethird handover request message generated by the processor to the basestation, wherein the receiver is further configured to receive a thirdhandover request response message from the base station, wherein thethird handover request response message comprises information about a CSdomain radio resource allocated by the base station to the UE for thehandover to the CS domain of the 2G or 3G network; the processor isfurther configured to generate a first handover response messageaccording to the first handover request message received by thereceiver, wherein the first handover response message comprises theinformation about the CS domain radio resource; the transmitter isfurther configured to send the first handover response message generatedby the processor to the MME.
 14. The device according to claim 13,wherein the receiver is further configured to receive a handovercomplete message from the UE, wherein the handover complete messageindicates that the UE has already completed the handover from the LTEnetwork to the CS domain of the 2G or 3G network.
 15. The deviceaccording to claim 14, wherein: the receiver is further configured toreceive a first CS call connection setup request message from the UE,wherein the first CS call connection setup request message is used torequest the MSC to generate a first session.
 16. The device according toclaim 14, wherein: the transmitter is further configured to send asecond CS call connection setup request message to the UE, wherein thesecond CS call connection setup request message is used to request theUE to generate a second session.
 17. The device according to claim 15,wherein: the transmitter is further configured to send a session releasemessage to the UE after the receiver receives the first CS callconnection setup request message, wherein the session release message isused to request the UE to release a third session generated by the UE ina process of the handover from the LTE network to the CS domain of the2G or 3G network.
 18. The mobile switching center according to claim 14,the first handover request message is further used to indicate that theUE supports the handover from the LTE network to the CS domain of the 2Gor 3G network.
 19. A device, comprising: a transmitter, configured tosend a request message to a mobility management entity (MME), whereinthe request message is used to request circuit switched fallback (CSFB);a receiver, configured to receive a handover command from an evolvedNodeB (eNodeB), wherein the handover command is used to instruct thatuser equipment (UE) be handed over from a Long Term Evolution (LTE)network to a circuit switched (CS) domain of a second generation (2G) orthird generation (3G) network for the CSFB, and the handover commandcomprises information about a CS domain radio resource allocated by the2G or 3G network to the UE for the handover from the LTE network to theCS domain of the 2G or 3G network; and a processor, configured to handover from the LTE network to the CS domain of the 2G or 3G networkaccording to the handover command received by the receiver; and generatea session after the UE is handed over from the LTE network to the CSdomain of the 2G or 3G network; wherein the transmitter is furtherconfigured to send a CS call connection setup request message to amobile switching center (MSC), where the CS call connection setuprequest message is used to request the MSC to generate a session; andthe processor is configured to generate the session after the receiverreceives the CS call connection setup request message sent by the MSC.20. The device according to claim 19, wherein the transmitter is furtherconfigured to send a handover complete message, wherein the handovercomplete message indicates that the UE has already completed thehandover from the LTE network to the CS domain of the 2G or 3G network.